Radar Question?

[samuel stone]

what's this ring of precip toatal on the Little Rock, AR Radar.

Go to "Storm Total" on the left side of the screen.

http://www.srh.noaa.gov/ridge/lzk.shtml

Did the precipitation actually fall in this pattern, or am I right in thinking that it has somthing to do with the radar reading at a higher level of the atmosphere further away from the radar site? Or is somthing completely different?

Appreciate any insight.

 

[B Ozanne]

Its possible that the precip fell in that pattern. Very unlikely though!

It has to do with the angle the radar is at. The father you get from the radar site the higher its beam reaches into the atmosphere. One possibility is that at those farther ranges the radar was "seeing" frozen precip that was just beginning to melt. This gives false returns, and it appears to be heavy precip. The radar thinks that heavy precip fell at the surface then calculates heavy rain totals. This is known as bright banding.

 

[samuel stone]

yeah, I figured it had somthing to do with the radar beam reading higher in the atmosphere further from the radar sight. I have seen similar things in the past while browsing through rain totals. It is a somewhat interesting phenomenon, but at the same time It can be rather anoying when trying to get a good picture of rainfall amounts, I suppose doppler estimations aren't a good way to recieve accurate rainfall amounts.

Thanks for the insight.

 

[Jeff Snyder]

I'd say that it could have been two things (disregarding the possibility that the precip naturally fell in that pattern) -- bright banding and evaporation. As snow / frozen precip approaches the -5C level (or even colder than that), the snow becomes sticky, leading to the possibility of very large snowflakes. The outer parts of the snowflakes begin to melt, resulting in liquid-covered, large volume particles (ice has lower density than water, so a particle of a given mass with have a higher volume in the ice stage than in the liquid stage). Since liquid water liquid reflects radar energy much better than ice, these relatively large, liquid-coated particles usually result in anomalously high reflectivities. This results in the typical "bright-band" effect seen near the freezing level.

The second possibility is that the very dry low-level air led to significant evaporation as the precip fell though the lowest few kilometers. If the precip wasn't very deep, then the higher beam heights (and thus locations farther from the radar given a fixed elevation angle) would have relatively low reflectivity. Meanwhile, locations near the radar on a set elevation would be sampled (by the radar) near the surface, where the dry air is evaporating precip and leading to reduced reflectivity. So, low reflecitivites would characterize the locations far from the radar (high beam height) and very near the radar (low beam height).

The shape of the storm-total precip looks like the result of classic bright-banding, but the second possibility may have aided in this as well. I know there was a lot of evaporation occurring from dry low-level air for precip in much of OK last night, though I haven't looked at an Arkansas surface map today.

 

[B Ozanne]

I agree with Jeff. I'd say most of it was bright banding, with a little bit of evaporation. There is just too much precip for it all to be related to evaporation.